Bearings with plastic-based overlays are known as single-layer, two-layer or three-layer composite materials. Single-layer bearing materials include solid plastic bearings. Two-layer bearing materials include bearings with an outer metallic backing and directly applied or adhered plastics. Three-layer bearing materials include bearings comprising a backing material, a porous layer, and a plastic-based covering layer formed in the pores. Each of these bearings may be useful in applications where the use of externally supplied lubricants may be difficult or undesirable.
With regard to the three-layer bearing materials, many different types of plastic bearing materials have been applied to a backing material, such as steel, having a porous metallic layer. The porous metallic layer may comprise bronze or copper particles sintered to a steel backing. The plastic bearing material may comprise a base polymer and filler particles. A useful plastic bearing material may include polytetrafluoroethylene (PTFE) as the base polymer.
The development of PTFE lined composite dry bearing material was begun by Glacier Metal Co. in 1948 and patents were granted for the materials during the 1950's. One of the materials developed was DU™. DU is a commercially successful bearing material combining dry wear properties of PTFE and the mechanical properties of conventional bearing material.
A process for impregnating a plastic bearing material into a porous metallic layer on a metal backing includes spreading a paste or dry powder onto the porous metallic layer, and the paste or dry powder is compacted into the pores via rolling. The paste or dry powder may be made by mixing an aqueous dispersion of PTFE with a filler material together with an organic lubricant (such as a volatile organic compounds “VOC's”) and coagulating the dispersion to form a so-called “mush”. Once the plastic bearing material is compacted into the porous layer, the backing material can be heated using an induction furnace to drive off any residual water and lubricant in the plastic bearing material. Heating the backing material may also melt or sinter the PTFE particles in contact with the porous layer and/or the backing material. Driving off the residual water can limit the thickness of the layer which may be formed above the porous layer (“overlay”) due to blistering which can occur when thicker layers are attempted. Further, the evaporation of VOC's from the compacted plastic bearing material can degrade the integrity and porosity of the overlay. Too thin of an overlay can lead to limited wear resistance.
Alternatives to the method of spreading a coagulated polymer dispersion onto a porous metallic layer have been developed, such as creating a tape of PTFE for impregnation into a porous metal layer. Although PTFE may be classified as a thermoplastic, it does not melt like other typical thermoplastics. At its transition temperature, PTFE changes to a rubber-like state that may be unsuitable for melt processing.
In some processes, when a PTFE tape is impregnated into a porous layer, the backing material and the porous layer may be heated, and a portion of the overly may be left in an unsintered form. In other words, the PTFE may not be in a continuous consolidated form, thereby potentially giving poor wear properties to bearings made using such a method.
A method of producing sintered PTFE tape is to press and sinter a cylindrical block of the polymer, with or without the incorporation of fillers, and to skive off a tape from the surface of the cylinder. Alternatively melt extruded tape comprising substantially similar portions by volume of polyphenylene sulphide (“PPS”) and PTFE may be impregnated into a heated porous layer disposed upon a heated backing material, as described in U.S. Pat. No. 5,665,825. The melt extrusion process is conducted at a temperature high enough to melt the PPS but low enough to avoid sintering the PTFE. This process can form interpenetrating networks of PPS and PTFE. Alternatively, the use of a conveyor and compressing roller system to produce PTFE based sheeting of tape is known. These forms of PTFE tape may not suitable for roll impregnation into porous metal sinter, because in some instances the PTFE may be sufficiently strong even above its transition temperature such that the rolling process may damage the porous metal sinter.
As a result, there is a need for three-layer bearing materials having greater overlay thickness that may extend bearing life by improving wear and erosion resistance while maintaining low friction. Thicker overlays may also allow for subsequent boring operations to be carried out in wrapped bushes. Further, there is a need for eliminating the use of VOC's in plastic bearing materials applied to a porous layer since avoiding the use of VOC's can improve the integrity and porosity of the overlay.